A significant aspect of state-of-the-art automotive engine control pertains to so-called on-board-diagnosis of various engine components or sensors, particularly when improper operation of such components or sensors can adversely influence the engine emission controls. In the case of an engine cooling system, the diagnosis involves determining if the engine thermostat is operating correctly, and if the engine coolant temperature sensor is providing an accurate measurement. If the diagnosis reveals faulty operation of the thermostat or engine coolant sensor, the engine controller stores the fault information and activates a "check engine" indicator to advise the driver that the vehicle should be serviced soon.
While any component or sensor could theoretically be diagnosed through the use of additional sensors, such additional sensors significantly increase the cost of implementing the diagnostic function, and raise concerns over the reliability of the additional sensors themselves. Accordingly, there is a strong desire to implement the diagnostic functions using existing sensor information only. Further, it must be recognized that engine operation varies widely depending on both the driver and the environment. In the case of the engine cooling system, for example, the thermostat operation and the measured coolant temperature are influenced by a number of factors, such as engine fueling, driving schedule, heater operation, ambient air temperature, engine speed, vehicle speed, etc. Diagnosing methods that take many of these factors into account can be very complex and are typically very difficult to calibrate properly.
Accordingly, what is needed is a method of reliably diagnosing the proper operation of an engine cooling system that does not involve the use of additional sensors and that is easy to calibrate.